In the operation of hydraulic pumping units useful in oil well applications, the bridle and attached well polish rod move upwardly and downwardly in direct response to the flow of hydraulic fluid into and out of hydraulic cylinders having piston rods therein which are attached to the bridle and well polish rod. Thus, a pumping unit forces hydraulic fluid from a tank through an open hydraulic control valve into the bottom of a hydraulic cylinder to raise the piston rod therein which causes the attached bridle and well polish rod to be pulled upwardly. Fluid continues to be pumped into the hydraulic cylinder until an upper limit switch closes the hydraulic control valve and vents the cylinder to the tank. Under these conditions, the weight of the bridle and polish rod pulls the piston down causing fluid to flow out of the hydraulic cylinder and into the tank.
In most hydraulic pumping systems, pilot operated relief valves are used to control piston rod movement. However, pilot operated valves offer two significant disadvantages. Due to close tolerances within the valves, they are subject to malfunction due to the accumulation of foreign material within the valve or due to expansion or contraction resulting from thermal transients to which the valve is exposed. In addition, the design of these valves is such that the transition from pressure to vent or vent to pressure positions is rapid, thus producing a sudden hydraulic shock due to the pressure transient. This shock is extremely undesirable in the operation of the hydraulic pumping system.
U.S. Pat. No. 4,219,049-Skelly discloses a valve for use in a hydraulic system to feather the hydraulic shock associated with increasing hydraulic fluid flow by suddenly bringing on-line additional hydraulic fluid pumps. The Skelly valve comprises two separate valve body elements which are relatively rotatable on a common shaft. A first valve body element provides a pair of separated cavities, one of which is connected to a working outlet port and the other to a vent outlet port. The other valve body element has a plurality of inlet ports connectable to a plurality of separate hydraulic pump outlets. By relative rotation of the valve body elements one or more of the inlet ports can be placed into communication with the cavity connected to the working outlet port to vary the working hydraulic fluid flow.
U.S. Pat. No. 3,134,405-White et al discloses a valve for use in high pressure gas systems to provide a controlled rate of change of pressure differential and avoid shock as the high pressure upstream side of the valve is brought into communication with the lower pressure downstream side of the valve. This is accomplished by providing, in a ball valve, a relatively small diameter bleed passageway extending diametrically through the ball at a substantial angle to the relatively large diameter normal flow bore in the ball such that the small diameter bleed passageway communicates with the valve inlet and outlet ports to gradually equalize upstream and downstream pressure before the relatively large diameter bore allows full flow through the valve between the ports.
However, none of the prior art valves allows sufficiently selective control of the pressure equalizing flow area to permit smooth pressure transitions in all circumstances. It is therefore the purpose of the present invention to overcome previously encountered practical difficulties and to provide an efficient, simple and relatively inexpensive valve for use in hydraulic systems which permits a smooth transition of hydraulic fluid as the valve switches between pressure and vent positions.